There are previously known systems for controlling the heading of a vehicle by deflection of a steering actuator. For example, to steer an automobile along a road, a driver deflects a steering wheel by an angle required to generate a desired turning rate. When a desired heading is reached, the steering wheel is centered to reduce the turning rate to zero. However, when encountering a crown in the road, a steering bias angle must be applied to the steering wheel to maintain the automobile on the road.
A skipper steers a boat in much the same manner by rotating a rudder, operating a tiller, or otherwise changing a thrust angle of a propelling force. However, when encountering a crosswind, crosscurrent, or other seastate condition, a steering bias angle must be applied to the steering actuator to maintain the desired heading. Steering bias is particularly necessary in small boats, which are susceptible to heading changes caused by variations in wind, tide, waves, wake, crew-induced listing, and off-center outboard motor mounting positions. Marine autopilot systems typically implement the steering bias angle by employing some form of an integrator that accumulates an error signal in a closed loop control system. Such systems are referred to as having "auto-trim."
The integrator is typically implemented by an electronic analog or digital integrator that is connected within the control loop that carries the heading or a heading error signal. The actual heading is typically generated by an electrical "flux gate" compass. Such control systems are referred to as position control systems and require some form of steering actuator angle sensor to close the loop. Unfortunately, it is not a straightforward task to adapt such a sensor to tiller-steered outboard motors, and none is known to have provisions for such a sensor. Moreover, existing position control-based marine autopilot systems have stability problems, as indicated by user controls to adjust for seastate conditions, rudder response, and damping.
Prior closed loop autopilot systems exist for watercraft that are steered by a wheel that is coupled to a cable or a hydraulic cylinder to turn a rudder or propulsion system. The wheel is readily adapted to include an actuator angle sensor. Commercially available closed loop autopilot systems that are adaptable to a cable or hydraulic steering system and have a seastate adjustment include the Navico Power Wheel PW5000, Benmar Course Setter 21, Furuno FAP-55, Robertson AP Series, Cetrek 700 Series, Si-Tex Marine Electronics SP-70, and Brooks and Gatehouse "Focus" and "Network" model autopilot systems. Some of the above-described autopilots are adaptable to inboard/outboard hydraulic steering systems, have handheld wired-remote control units, and include a built-in or remote flux gate compass.
A well-known provider of marine autopilot systems is Autohelm of Hudson, N.H., which manufactures the SportPilot, ST1000, ST4000, and ST5000 model autopilots. The Autohelm autopilots are adaptable to tiller, cable, or hydraulic, steering actuators, have four levels of steering trim adjustment, adaptive and programmable seastate adjustments, and variable rudder gain and damping adjustments.
The hydraulic steering systems employed in larger watercraft are typically high-pressure continuous flow types that employ expensive servovalves or modulated solenoids. In contrast, hydraulic steering systems for smaller watercraft are typically "hydrostatic" types that are smaller, simpler, and less expensive.
Some autopilot systems, particularly those for smaller watercraft, employ relatively simple "bang-bang" servo steering controllers. Unfortunately, such steering controllers consume excessive power typically require "dead-band," damping, rudder gain, and seastate adjustments. In small watercraft that typically have only a single 12-volt battery, power conservation is an important factor in ensuring reliable operation of running lights, radios, navigation equipment, water pumps, vent fans, and starter motors.
What is needed, therefore, is an automatic steering system for small watercraft that employs a self-trimming control system that does not require a steering actuator angle sensor or a seastate control for accurately and stably steering an outboard motor with a simple low power-consumption positioning system.